For many applications, the grain size of titanium alloys must be reduced. However, the processes for reducing grain size in titanium alloys requires a number of iterative processing steps, which is disadvantageous, both in time and expense. The sequence of TMP steps conventionally used to manufacture semi-finished and finished products in these titanium alloys is shown in FIG. 1 along with the microstructures that are produced by these steps. This involves the three general processes of ingot breakdown, conversion, and finishing (FIG. 1(a)). The objective of ingot breakdown is to break down the coarse as-cast microstructure and obtain a lamellar structure with a refined prior beta grain size. Conversion processing involves conversion of ingots into mill products (e.g. billets, plates, and rods) with a concurrent breakdown of the lamellar microstructure (FIG. 1(b)) into equiaxed grains (FIG. 1(c)). This is achieved by extensive deformation (>75% reduction) using a process such as cogging, illustrated in FIG. 1(d). Since the amount of deformation that the billet can withstand without damage in a single cogging step is much less than this amount, many iterations of this mechanical working step are required in the α+β phase field. Finishing involves either α+β or β processing with appropriate heat treatment to obtain the desired final microstructure. While the lamellar microstructure produced after ingot breakdown exhibits high strength and fracture toughness, equiaxed microstructures produced after conversion possess excellent ductility and resistance to crack initiation under low-cycle fatigue loading, each of which are necessary for fracture-critical structural components. Thus, the ingot breakdown and conversion steps in the TMT sequence used for the majority of titanium alloys are critical, and consume a significant amount of time (14-16 hours). A need exists therefore for an improved method for microstructural refinement that reduces or eliminates the number of iterative processing steps, thus reducing the cost and the lead-time in component manufacture while maintaining the necessary microstructural control. Such a process would provide improved microstructural refinement and make the manufacture of titanium components more affordable.
Superplasticity is the ability of the material to undergo large plastic deformation in tension (above 200% elongation) without failure and is typically exhibited by materials with fine grain size (<10 μm). Titanium alloys do not exhibit fine-grained superplasticity at high temperatures in the monolithic beta phase field due to rapid grain growth. A need exists for the ability to achieve superplasticity in titanium alloys at higher temperatures; thus enabling efficient forming of intricate near-net or net shapes with enhanced mechanical properties using smaller capacity presses. A need further exists for achieving superplasticity at higher strain rates compared with conventional superplasticity, which would considerably increases the production rates of wrought titanium alloy products.